Pay now, or trust in the future
If you put aside a pound a year, and invest it, after 20 years, you could have £30.
Multiply by a billion, and you've got the latest piece of Alex Salmond's vision for an independent Scotland.
It's simple, and an example of how you could use surplus tax revenue from, say, oil.
Plus, it's a return to a once well-worn theme for the Scottish government, set out once again by the first minister on Wednesday in London.
As David Cameron prepared to head to Scotland to set out his positive vision for the United Kingdom remaining united, Alex Salmond was citing the example of Norway.
It used its oil funds to pay down debt to zero, and it's spent more 20 years building up a giant nest egg - or more like a golden goose that keeps on laying.
The Scandanavians' global pension fund is worth more than £300bn, with which it owns 2% of the world's stock markets, making strategic and ethical investments around the world. And very impressive it is too.
But, as I found out when I went to Oslo, it required a level of political discipline not to spend the money in the short term, to limit the extent to which earnings can be taken from the fund, it needed an explicit expression of trust in future generations and the political maturity to accept that share prices can go down as well as up, with white knuckle consequences in recent roller-coaster years.
Oiling the wheels
The argument that North Sea oil should be used for the same purpose has long been used by Scottish nationalists, but it doesn't have to be their monopoly.
Even within the current United Kingdom, the funds could have been set aside, instead of being mixed in with all other tax revenues by the Treasury, and used to fund current budget priorities.
So it's not just Scotland that could have had a Norwegian-style fund if it had the powers and the foresight to build one up from the 1980s. That's true of the UK too.
But Westminster governments chose not to.
So one question to bear in mind when looking back with hindsight; was the 1980s a time when Scottish politicians seemed willing to forego current expenditure in favour of creating a trust fund, if only they could?
If memory serves correctly, the instinct and priority then was to protect old industries, and the communities that depended on them, and there would have been the temptation to use those oil funds as subsidy.
And what of Alex Salmond's forward-looking plan? He points out oil may be running out, but more slowly than previously thought, and at a higher value per barrel.
Fuelled by Middle East tensions, benchmark Brent Crude nearly hit $120 per barrel on Wednesday.
So that's where he gets his suggestion that £1bn could be set aside by an independent Scotland each year, resulting in £30bn after 20 years.
It is merely illustrative. It doesn't seem to be a commitment.
And how could it happen? Well, if Norway is the example to follow - or indeed, common sense - the paying off of debt would be necessary before building up a fund.
The UK now has £1 trillion in net public sector borrowing.
If an independent Scotland gets its share of that - either population-based or the relative size of its economy - it could expect to take on slightly more than £80bn of debt.
That's a lot. It's not significantly, proportionately different from the debt carried currently by the UK, so although interest payments cost more this year than the entire defence budget, it ought to be manageable.
But then there's plenty argument to be had about whether Scottish debt could be financed at the same low rates the UK currently enjoys.
It could face an interest rate premium due to its lack of an independent credit history, and its politicians' appetite, from both main parties, for more borrowing to boost government spending.
The lesson from bond markets around Europe is that higher borrowing comes at the price of higher risk of default.
But it can also be pointed out, citing Moody's recent negative outlook on UK government credit, that not enough borrowing can have a similar effect through stunting growth.
So if an independent Scotland eventually made the sacrifices to pay down the debt to zero, it would only then face continuing sacrifices to put money aside if it's to amass £30bn within 20 years.
And while there are some years when Scotland turns a fiscal surplus - including a geographic share of oil revenue and excluding capital spending - there haven't been spectacular surpluses for many years.
Except in a rare good year, that oil revenue would be needed to sustain the current level of spending on public services and welfare payments.
Much of this may sound familiar to those who have followed the debate down the years.
But one factor that is less familiar is the external challenge to the prevailing story of Scotland as subsidy-junkie.
You'd expect Alex Salmond to use his lectures to English audiences to challenge such a notion, with an emphasis on Scotland's economic and fiscal strengths.
But the calculations from the Centre for Economics and Business Research earlier this week make an interesting independent contribution of relevance to independence.
The London think tank wasn't asking what might happen if Scotland became independent.
It was trying to compare revenue and expenditure around the UK nations and regions. It found wide disparities.
In London, tax accounts for 45% of GDP. In Northern Ireland, at the other end of the spectrum, a poorer economy pays tax at only 28% of the income it generates.
CEBR reckons also on the share of GDP spent by government. London comes in at 35% of the income it generates.
Northern Ireland is again at the other end of the spectrum, on 67%, with Wales less than 1% behind.
The gulf between these figures show the extent to which money appears to be redistributed around the UK according to the strength of regional and national economies, and their pull on government spending.
London is reckoned to pay 20 percentage points more than it gets back in spending.
This reckoning of redistribution means Northern Ireland gets back 29% more than it gives, Wales 26%, north-east England 22%.
Scotland was found to spend 53% of its GDP, while tax only brought in 43%.
So clearly, it's subsidised to the tune of 10%, right? Wrong. Not by this calculation. It looks at the relative position.
With a bit of CEBR economists' jiggery-pokery, it reckons the UK as a whole requires to borrow so much that its deficit amounts to a 10% overall "subsidy".
That way, Scotland's position is the only one of the UK's nations and regions that comes out in a sort of balance.
Those for the union can take from this an illustration of the insurance policy and spreading of risk and income that nations and regions can rely upon as part of a larger nation that redistributes funds.
The nationalist side can be at least as cheered to find an independent survey of finances that suggests Scotland is well able to hold its own, at least under current circumstances.
It will be all the more cheering to find this same think tank made waves in recent years by rubbishing Scotland's over-weening public sector.
At first, it bracketed Scotland with North Korea and Cuba, and then warned the nation was at risk of being overtaken by fast-growing countries such as South Korea, Poland and, yes, even Greece.
So, if you choose to believe the latest figures, this is some sort of progress for Scotland, and in the quality of the debate about its future.